1. Field of the Invention
The present invention relates generally to melting apparatuses for melt decontamination of radioactive metal waste and, more particularly, to a melting apparatus which melt-decontaminates different kinds of metal waste generated from nuclear facilities, especially, facilities for processing or producing nuclear fuel, thus forming a decontaminated ingot from which radioactive contaminated slag is removed so that the decontaminated ingot can be recycled.
2. Description of the Related Art
Industrial waste, the principal ingredients of which are ferrous metals such as stainless steel and carbon steels, is perceived as being a valuable resource, and its recycle ratio is quite high compared to other kinds of waste.
Generally, the purpose of recycling waste is to cope with a dearth of natural resources and the problem of environmental pollution such as air, water or soil and other kinds of pollution. Particularly, although metal waste is a kind of waste which must be reprocessed to be recycled, given that the cost of recycling metal waste is markedly less than that of using natural resources to produce a product, it is a big loss in terms of protection of the environment or in the economic sense that metal waste is discarded rather than being recycled.
Metal waste which is generated from nuclear facilities can also be reused by a recycling process in the same manner as other industrial metal waste. However, there is the possibility of such metal waste having been made radioactive by artificial neutron irradiation or been contaminated by radioactive substances used in the nuclear facilities. If such metal waste is recycled to produce products without adhering to appropriate regulations and the products are put on the market, an unspecified number of the general public may be exposed to radiation by the contaminated products. Therefore, all of the metal waste generated in radiation controlled areas in a nuclear facility is targeted for control. However, despite the case where the concentration of radio-nuclides in metal waste is infinitesimal so that it barely has any radiological effect on the public and the environment, if the same regulations are applied to the case, economic and social costs may be unnecessarily incurred. Hence, in the nuclear relevant act of South Korea, only when the concentration of radio-nuclides in metal waste is below a clearance level (a clearance limit), in other words, only when the radiological effect on the public and environment attributable to recycling of metal waste is below a disposal criterion that complies with the nuclear relevant act, is metal waste allowed to be discarded (or recycled). Furthermore, a related regulatory agency strictly requires radiation safety management and evaluation of radiological harm so that the radiological effect to the public and environment which is caused by clearance can be minimized.
For example, it is expected that metal waste, such as a filter frame, a powder drum for a natural uranium, nuts, bolts and metal scraps, which were used in facilities for processing and producing nuclear fuel are contaminated with uranium compounds such as UO2, UO2F2 or U3O8. Therefore, such metal waste is regarded as radioactive waste which becomes a target of control, but if the concentration of the source of radiation pollution in the metal waste is below the clearance level, the metal waste is exempted from the regulations and is allowed to be disposed of by a recycling method or the like.
If the shape of metal waste is that of a planar plate or the like which has a comparatively simple geometrical shape and a smooth surface, it can be recycled only by surface decontamination. Radioactive concentration is measured in real time during the decontamination process by a combination of a direct measurement method using a surface contamination monitor which is used in a nuclear fuel processing site and an indirect measurement method using a smear method. However, in the case of metal waste such as a nut or bolt which has a complex geometrical shape, it is impossible to directly measure its surface contamination, or it is difficult to use the smear measurement method. Therefore, such metal waste creates a lot of difficulties during decontamination or radiological monitoring processes.
For the above reasons, a melt decontamination method is used. If metal waste is heated to a high temperature and melted, not only can radioactive substances in the metal be evenly distributed in a medium, but nuclear fuel material which is the source of the pollution can also be contained in slag on molten metal. The melt decontamination method uses these characteristics. If metal waste that has a complex structure which makes surface decontamination and direct radiation measurement difficult is processed by the melt decontamination method, the volume of the metal waste can be reduced, and uranium substances can be easily removed from a metal medium before the decontaminated metal waste is disposed of.
Hitherto, a lot of research into a technique for melt-decontaminating metal waste that contains radioactive substances has taken place. Particularly, it has been reported that if the source of pollution is a nuclear fuel material (uranium radio-nuclide), most of the source of radiation pollution is contained in slag when melted. Although the decontamination effect is different depending on the initial conditions of contamination, the kind of melting additive and operation conditions such as the type of a melting furnace, the amount of uranium that is contained in slag when melt-decontaminating metal waste is over 1,000 times the amount of uranium that is contained in an ingot. It has been reported that as the initial degree of contamination increases, such a tendency also increases.
For example, a system for melt decontamination of radioactive scrap metal was proposed in Korean Patent Registration No. 10-1016223. In this melt decontamination system, U-238, Ce-144, Cs-134, Cs-137, Sr-89, Sr-90, Ni-63, Co-58, Co-60, Cr-51, etc. are the target nuclides to be decontaminated. The system melt-decontaminates metal waste polluted by radioactivity generated in nuclear facilities, thus forming a decontaminated ingot from which slag that contains radioactivity is removed. The decontaminated ingot is recycled, and the slag that contains radioactivity is disposed of as radioactive waste.